With the improvement of people's living standard, the dietary structure is changed, the incidence of cardiovascular diseases becomes higher and higher, and the coronary heart disease caused by cardiovascular stenosis has become a main disease dangerous to human health. The percutaneous transluminal coronary angioplasty procedure is the primary method of treating coronary atherosclerotic heart disease. Since 1977, through nearly three decades of continuous development and improvement, the interventional treatment for coronary heart diseases has been constantly improved in the technological level and has totally experienced three eras, i.e., Balloon Dilatation (PTCA), Bare Metal Stents (BMS) and Drug Eluting Stents (DES).
At present, coronary stents commonly used in the clinic are classified into two categories, i.e., bare metal stents and drug eluting stents. Although DESs may reduce the rate of restenosis and repeat revascularization, but existing polymer carrier drug eluting stents still have certain limitations. These limitations are mainly manifested as late and very-late stent thrombosis, endothelial healing delay and late catch-up of luminal loss. However, the main reason is inflammatory reaction caused by polymer carriers. This problem and effective means for solving this problem have always been widely disputed in international research fields. When the restenosis occurs, a stent is unable to be implanted secondarily as it is disadvantageous to the late angioplasty. Therefore, it is desirable to research and develop biodegradable stents.
Biodegradable stents have become a hotspot of research and development. For example, taking an iron-based alloy vessel stent for an example, by surface alloying the iron-based alloy stent, for example, nitriding from a surface to inside to form a readily-corrodible diffusion layer having high hardness, the strength of the stent is improved, the corrosion rate of the stent is quickened, and the absorption period of the stent is shortened. However, the stent is very likely to form a dense ε-phase or γ′-phase compound layer (commonly known as a white bright layer) having high nitrogen content on its surface during nitriding. This compound layer has relatively stable chemical properties, is able to resist the corrosion of acidic or alkaline solution, and is difficult to be corroded in a human tissue. If the compound layer cannot be effectively removed during the process of manufacturing the iron-based alloy vessel stent, the corrosion rate of the stent will be greatly affected, and the absorption period of the stent is thus prolonged.
At present, removing this compound layer may be performed by mechanical polishing, electrochemical polishing, vacuum denitriding, and ion nitriding at a low nitrogen potential. However, these methods all have some defects.
Although the mechanical polishing may quickly remove the white bright layer, a fine implant medical instrument, for example, a very small strut of a vessel stent, has very strict dimensional tolerances (e.g., a precision of 5 μm). The dimensional precision of such a medical instrument cannot be ensured if the mechanical polishing is used, that is, the fine medical instrument is unable to employ this method.
The electrochemical polishing needs to use strong acids or other solutions, so it is very likely to result in the corrosion defect of the surface of a nitriding layer of a medical instrument after the compound layer is removed. Meanwhile, the surface of a diffusion layer of the fine medical instrument may become bright and flat only after the removal amount of polishing (a difference in the thickness of the medical instrument before and after polishing) reaches 40 μm, so that the compound layer closest to the surface will be removed, and a part of the diffusion layer will also be removed. As a result, the remaining diffusion layer will be very thin, which is disadvantageous to the quality control of a thin-wall portion (e.g., a strut of a vessel stent) of the medical instrument.
The vacuum denitriding may facilitate the change of a phase structure by changing ambient pressure and temperature, thereby realizing the desorption (retroaction of adsorption) of nitrogen in the compound layer and the diffusion layer. If the temperature holding time is long enough (6-9 hours), the nitrogen concentration of the surface may be reduced so that the compound layer is reduced or eliminated. However, actually, as the required temperature holding time is too long, the escape of nitrogen atoms will influence the diffusion layer of a vessel stent and thus reduce the surface hardness, but there are still few compound layers on the surface of the vessel stent. Therefore, this method is unable to really effectively remove the compound layer, and will reduce in turn the performance of the stent.
Theoretically, during nitriding at a low nitrogen potential, when the actual nitrogen potential does not exceed a particular threshold during nitriding, the compound layer will not be formed. However, this method greatly restrains the permeation rate of nitrogen atoms, so the nitriding efficiency is greatly reduced. As a result, a very long temperature holding and diffusion time is required to form a desired nitriding layer. Due to the fact that the dispersed phase in the nitriding layer is gathered and grown, both the hardening effect and galvanic corrosion effect of the dispersed phase are weakened, and the basic performance of the iron-based alloy medical instrument is thus reduced.
Therefore, it is necessary to provide a manufacturing method, which gives consideration to both product performance and manufacturing efficiency, which quickly and effectively removes a compound layer formed on an iron-based alloy medical apparatus after nitriding, and which meets the performance requirements of a biological medical apparatus.